Electronic ballast having a stable reference voltage and a multifunction input for soft dimming and ON/OFF control

Abstract

An electronic ballast for use in illuminating a lamp includes a voltage reference generator that uses a plurality of current amplifiers and resistors having substantially identical resistance characteristics to remain stable in response to temperature variations and despite resistance process dispersion. The reference voltage generator further includes an ON/OFF controller and a dimming function that may be controlled via a single input terminal. Additionally, the dimming function uses a capacitor to prevent abrupt changes in an input signal from causing abrupt changes in a feedback signal that controls an output frequency of the ballast.

What is claimed is:  
     
       1. A current amplifier for use in an electronic ballast, the current amplifier comprising: 
       a current source;  
       a differential amplifier coupled to the current source, the differential amplifier including first and second resistors and first and second transistors, wherein an emitter terminal of the first transistor is coupled to the first resistor and an emitter terminal of the second transistor is coupled to the second resistor;  
       a selection circuit including third and fourth transistors, wherein an emitter terminal of the third transistor is coupled to an emitter terminal of the fourth transistor and the emitter terminals of the third and fourth transistors are coupled to a base terminal of the second transistor;  
       a first current mirror;  
       a second current mirror coupled to the first current mirror and to a collector terminal of the first transistor; and  
       a third current mirror coupled to the first current mirror and to a collector terminal of the second transistor.  
     
     
       2. The current amplifier of claim  1 , wherein base terminals of the third and fourth transistors are non-inverting input terminals of the selection circuit and wherein collector terminals of the third and fourth transistors are coupled to a ground potential. 
     
     
       3. The current amplifier of claim  1 , wherein the first and second resistors have substantially identical resistance characteristics. 
     
     
       4. The current amplifier of claim  1 , wherein the first and second transistors have substantially identical electrical characteristics. 
     
     
       5. The current amplifier of claim  1 , wherein the third and fourth transistors have substantially identical electrical characteristics. 
     
     
       6. The current amplifier of claim  1 , wherein the second current mirror includes fifth and sixth transistors and the third current mirror includes seventh and eighth transistors and wherein the fifth through eighth transistors have substantially identical electrical characteristics. 
     
     
       7. The current amplifier of claim  1 , wherein first current mirror includes ninth and tenth transistors having substantially identical electrical characteristics. 
     
     
       8. The current amplifier of claim  1 , wherein the first current mirror includes third and fourth resistors having substantially identical resistance characteristics. 
     
     
       9. A reference voltage generator for use in an electronic ballast, the reference voltage generator comprising: 
       a comparison voltage generator having a soft start current source and a soft start capacitor coupled to the soft start current source;  
       a first amplifier having a first current amplifier, wherein the first current amplifier has first and second non-inverting input terminals and a first inverting input terminal and wherein one of the first and second non-inverting input terminals is coupled to the soft start capacitor and the other one of the first and second non-inverting input terminals is coupled to a first amplifier reference voltage;  
       a first current source having a first current mirror, first and second resistors, and a first transistor, wherein a first output terminal of the first current mirror is coupled to a collector terminal of the first transistor and a second output terminal of the first current mirror is coupled to the second resistor and wherein a base terminal of the first transistor is coupled to an output terminal of the first current amplifier and the first resistor is coupled between a ground potential and the inverting input terminal of the first current amplifier;  
       a capacitor charger that compares a voltage across the soft start capacitor to a first comparison reference voltage and charges a dimming capacitor based on the comparison;  
       a second amplifier having a second current amplifier, wherein the second current amplifier has third and fourth non-inverting input terminals and a second inverting input terminal and wherein one of the third and fourth non-inverting input terminals is coupled to a dimming voltage and the other one of the third and fourth non-inverting input terminals is coupled to a second amplifier reference voltage and wherein an output terminal of the second current amplifier is coupled to the dimming capacitor and the second current amplifier selects the smaller of the dimming voltage and the second amplifier reference voltage to control a charging characteristic of the dimming capacitor;  
       a second current source that supplies a current proportional to the voltage selected by the second current amplifier and the charging characteristic of the dimming capacitor; and  
       an ON/OFF controller that controls the operation of a lamp system based on a comparison of the dimming voltage to a second comparison reference voltage.  
     
     
       10. The reference voltage generator of claim  9 , wherein the first and second current amplifiers each comprise: 
       a current source;  
       a differential amplifier coupled to the current source, the differential amplifier including third and fourth resistors and second and third transistors, wherein an emitter terminal of the second transistor is coupled to the third resistor and an emitter terminal of the third transistor is coupled to the fourth resistor;  
       a selection circuit including fourth and fifth transistors, wherein an emitter terminal of the fourth transistor is coupled to an emitter terminal of the fifth transistor and the emitter terminals of the fourth and fifth transistors are coupled to a base terminal of the third transistor;  
       a first current mirror;  
       a second current mirror coupled to the first current mirror and to a collector terminal of the second transistor; and  
       a third current mirror coupled to the first current mirror and to a collector terminal of the third transistor.  
     
     
       11. The reference voltage generator of claim  10 , wherein base terminals of the fourth and fifth transistors are non-inverting input terminals of the selection circuit and wherein collector terminals of the fourth and fifth transistors are coupled to a ground potential. 
     
     
       12. The reference voltage generator of claim  9 , wherein the first and second resistors have substantially identical thermal characteristics and resistance process dispersions. 
     
     
       13. The reference voltage generator of claim  9 , wherein the capacitor charger comprises: 
       a first comparator having an inverting input terminal coupled to the soft start capacitor voltage and a non-inverting input terminal coupled to the first comparison reference voltage;  
       a fast charger coupled to an output terminal of the first comparator that provides a charging output based on a comparison of the soft start capacitor voltage and the first comparison reference voltage; and  
       a dimming capacitor coupled to an output terminal of the fast charger, a base terminal of a second transistor and the output terminal of the second transistor.  
     
     
       14. The reference voltage generator of claim  9 , wherein the first amplifier reference voltage is less than the first comparison reference voltage. 
     
     
       15. The reference voltage generator of claim  9 , wherein the second current source comprises: 
       a second current mirror having first and second output terminals that output currents of equal magnitude;  
       a second transistor having an emitter terminal coupled to an output terminal of the second current mirror and a base terminal coupled to the output terminal of the second current amplifier; and  
       an adder having first, second and third terminals, wherein the first terminal is coupled to the second current mirror, the second terminal is coupled to a reference current and the third terminal is coupled to the first resistor, and wherein the adder supplies a current to the first resistor based on a difference between the current output by the second current mirror and the reference current.  
     
     
       16. The reference voltage generator of claim  9 , wherein a magnitude of the first comparison reference voltage determines a soft dimming starting point. 
     
     
       17. A ballast for use in a lamp system, the ballast comprising: 
       a reference voltage generator having a terminal that controls ON/OFF operation and dimming of the lamp system, wherein the reference voltage generator further includes a dimming capacitor that prevents abrupt variations in a reference voltage and wherein the reference voltage generator includes a plurality of resistors having resistance characteristics such that the reference voltage is substantially stable in response to temperature variations and resistance process dispersion;  
       an oscillator that generates an alternating current signal based on a voltage output by a feedback circuit;  
       a half bridge converter; and  
       a half bridge converter driver that uses the alternating current signal to drive the half bridge converter.  
     
     
       18. The ballast of claim  17 , wherein the reference voltage generator comprises: 
       a comparison voltage generator having a soft start current source and a soft start capacitor coupled to the soft start current source;  
       a first amplifier having a first current amplifier, wherein the first current amplifier has first and second non-inverting input terminals and a first inverting input terminal and wherein one of the first and second non-inverting input terminals is coupled to the soft start capacitor and the other one of the first and second non-inverting input terminals is coupled to a first amplifier reference voltage;  
       a first current source having a first current mirror, first and second resistors and a first transistor, wherein a first output terminal of the first current mirror is coupled to a collector terminal of the first transistor and a second output terminal of the first current mirror is coupled to the second resistor and wherein a base terminal of the first transistor is coupled to an output terminal of the first current amplifier and the first resistor is coupled between a ground potential and the inverting input terminal of the first current amplifier;  
       a capacitor charger that compares a voltage across the soft start capacitor to a first comparison reference voltage and charges a dimming capacitor based on the comparison;  
       a second amplifier having a second current amplifier, wherein the second current amplifier has third and fourth non-inverting input terminals and a second inverting input terminal and wherein one of the third and fourth non-inverting input terminals is coupled to a dimming voltage and the other one of the third and fourth non-inverting input terminals is coupled to a second amplifier reference voltage and wherein an output terminal of the second current amplifier is coupled to the dimming capacitor and the second current amplifier selects the smaller of the dimming voltage and the second amplifier reference voltage to control a charging characteristic of the dimming capacitor;  
       a second current source that supplies a current proportional to the voltage selected by the second current amplifier and the charging characteristic of the dimming capacitor; and  
       an ON/OFF controller that controls the operation of a lamp system based on a comparison of the dimming voltage to a second comparison reference voltage.  
     
     
       19. A lamp system, comprising: 
       a power supply unit that supplies a lamp drive power;  
       a half bridge converter that uses a switch to supply the lamp drive power;  
       a lamp unit that uses the lamp drive power to emit light;  
       a reference voltage generator that uses a dimming capacitor to prevent abrupt changes in a reference voltage;  
       a feedback circuit that generates a feedback voltage based on a current flowing in the lamp unit and compares the feedback voltage to the reference voltage;  
       an oscillator that generates an oscillating signal based on a voltage output by the feedback circuit; and  
       a half bridge converter driver that receives the oscillating signal from the oscillator and supplies a current that changes direction to the half bridge converter.  
     
     
       20. The lamp system of claim  19 , wherein the reference voltage generator comprises: 
       a comparison voltage generator having a soft start current source and a soft start capacitor coupled to the soft start current source;  
       a first amplifier having a first current amplifier, wherein the first current amplifier has first and second non-inverting input terminals and a first inverting input terminal and wherein one of the first and second non-inverting input terminals is coupled to the soft start capacitor and the other one of the first and second non-inverting input terminals is coupled to a first amplifier reference voltage;  
       a first current source having a first current mirror, first and second resistors and a first transistor, wherein a first output terminal of the first current mirror is coupled to a collector terminal of the first transistor and a second output terminal of the first current mirror is coupled to the second resistor and wherein a base terminal of the first transistor is coupled to an output terminal of the first current amplifier and the first resistor is coupled between a ground potential and the inverting input terminal of the first current amplifier;  
       a capacitor charger that compares a voltage across the soft start capacitor to a first comparison reference voltage and charges a dimming capacitor based on the comparison;  
       a second amplifier having a second current amplifier, wherein the second current amplifier has third and fourth non-inverting input terminals and a second inverting input terminal and wherein one of the third and fourth non-inverting input terminals is coupled to a dimming voltage and the other one of the third and fourth non-inverting input terminals is coupled to a second amplifier reference voltage and wherein an output terminal of the second current amplifier is coupled to the dimming capacitor and the second current amplifier selects the smaller of the dimming voltage and the second amplifier reference voltage to control a charging characteristic of the dimming capacitor;  
       a second current source that supplies a current proportional to the voltage selected by the second current amplifier and the charging characteristic of the dimming capacitor; and  
       an ON/OFF controller that controls the operation of a lamp system based on a comparison of the dimming voltage to a second comparison reference voltage.